Most of this discussion assumes a mined coin is exchanged for the payout coin. The 20% reduction also occurs when the matured block
is the same currency as payout. In such cases there is no exchange. There's no use adding unnecessary variables to the problem.
I wish you success in finding the problem.
The theory is that the coin is exchanged behind the scenes from X to BTC and back to X for payout. Could very well be but the loss is too great and too consistent for that IMO.